DESCRIPTION: Widespread genomic instability is a hallmark of human tumors. The most obvious manifestation of genomic instability results in large-scale chromosomal rearrangements. These chromosomal rearrangements can lead to Loss-of-Heterozygosity (LOH) or novel gene fusions encompassing chromosome regions known to contain oncogenes and/or tumor suppressor genes. We have focused on the events that lead to chromosomal alterations in mitotic cells, which are likely to be initiated by aberrant recombinational repair following environmentally induced damage. The long-term goal of this research is to understand the biochemical and molecular processes that lead to the chromosomal aberrations found in human tumors. In the last granting period we discovered a complex association of recombination repair proteins. Two of the core players are hRAD51 and hMSH2, which BOTH appear to function as adenosine nucleotide regulated molecular switches. The concept and experimental tests of Adenosine Nucleotide Regulated Molecular Switch is novel and was developed in our laboratory. HRAD51 and hMSH2 were used to identify an additional 37-protein interactor (termed: proteome). It appears clear that the hRAD51-proteome is involved in the initiation of recombinational repair, while the hMSH2-proteome is likely to target intermediate-processing components to the recombination-initiation structures. Most of the proteome components display homology to yeast genes where both genetic and biochemical evidence underscores a role in recombinational repair. In this renewal grant application we propose to: I.) characterize the hRAD51 molecular switch function and regulation. II.) characterize the interaction domains and assembly of the hRAD51 proteome, III.) detail the recombination intermediate processing and targeting function of hMSH2-hMSH3.hMSH6 and IV.) characterize the interaction domains and assembly of the hMSH2 proteome. While these studies would appear broad and overwhelming in scope, we believe that unique collection of genes and reagents places us in a pivotal position to perform the proposed studies. It is likely that our work will contribute to understanding the processes involved in maintaining human chromosome stability and should provide framework for the design of more efficacious chemotheraputic and radiotherapeutic methodologies as well as preventative therapeutic modalities designed to control chromosome stability.